Method for injection compression molding

ABSTRACT

In a method of operating an injection compression molding machine having a stationary metal mold, a movable metal mold, an oil pressure actuator for advancing the movable metal mold toward the stationary metal mold to form a mold cavity therebetween with a predetermined compression clearance δ left between the movable and stationary metal molds, a heating cylinder adapted to engage with the stationary metal mold, a hopper for supplying a thermoplastic resin into the heating cylinder, a screw contained in the heating cylinder for injecting molten metal into the mold cavity, an oil pressure motor for rotating the screw and a piston cylinder unit for reciprocating the screw through the heating cylinder, the molten resin is filled in the mold cavity by advancing the screw and the advancement of the movable metal mold is started when the screw reaches a predetermined set position during an injection stroke so as to reduce the compression clearance δ and to increase the pressure acting upon the molten resin injected into the mold cavity.

BACKGROUND OF THE INVENTION

For molding, with an injection molding machine, such products as lensesand discs having a relatively large thickness and required to haveprecise configurations and small internal strains, an injection moldingmachine has been used capable of injecting a large quantity of moltenmaterial in each shot such that the quantity of the molten materialinjected into a mold cavity of a metal mold would not be deficient. Forthis reason, a method and apparatus have been used wherein a movablemetal mold is positioned in a stationary metal mold with a small gap orcompression clearance left therebetween, molten resin is filled in themold cavity defined between the movable and stationary metal molds, andthen the movable metal mold is advanced to close the gap or reduce tozero the compression clearance. However, this compression movement isstarted immediately after filling the molten resin into the mold cavity,or a predetermined time after completion of the filling of the moltenresin. For this reason, the compression starting time substantially lagsthe filling starting time with the result that the molten resin injectedinto the mold cavity solidifies starting from the contact surfacebetween the resin and the metal mold. Accordingly, when the compressionis continued under this condition, it is inevitable to create aninternal stress in the molded product.

In the prior art method wherein the compression clearance is reducedafter filling the molten resin, a large mold clamping force is necessarythus increasing the size of the injection molding machine. Where amolding machine requires a small mold clamping force but it is necessaryto increase the quantity of the injected resin per shot, the screw isrotated while being advanced toward the meta mold.

With this method, however, the molten resin is conveyed into the moldcavity only by the transfer force caused by the rotation of the screw sothat where the cavity has an intricated configuration, the resin cannotreach deep portions of the cavity. Moreover, as the filling speed of theresin is relatively low, there is the same defect that the resin beginsto solidify starting from the contact surface between the filled resinand the metal mold.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved method andapparatus capable of preventing creation of the internal strain in themolded product due to a lag of the compression motion of the movablemetal mold.

Another object of this invention is to provide an improved method andapparatus capable of manufacturing products of an intricatedconfiguration with an injection molding machine requiring a relativelysmall mold clamping force.

According to one aspect of this invention there is provided a method ofoperating an injection compression molding machine comprising astationary metal mold, a movable metal mold, an oil pressure actuatorfor advancing the movable metal mold toward the stationary metal mold toform a mold cavity therebetween with a predetermined compressionclearance δ left between the movable and stationary metal molds, aheating cylinder adapted to engage with the stationary metal mold, meansfor supplying a thermoplastic resin into the heating cylinder, a screwcontained in the heating cylinder for injecting molten metal into themold cavity, means for rotating the screw and means for reciprocatingthe screw through the heating cylinder, characterized in that the methodcomprises the steps of filling the molten resin in the mold cavity andstrrting advancement of the movable metal mold toward the stationarymetal mold when the screw reaches a predetermined set position during aninjection stroke so as to reduce the compression clearance δ and toincrease pressure acting upon the molten resin injection into the moldcavity.

According to another aspect of this invention there is providedapparatus for operating an injection compression molding machinecomprising a stationary metal mold, a movable metal mold, an oilpressure actuator for advancing the movable metal mold toward thestationary metal mold to form a mold cavity therebetween with apredetermined compression clearance δ left between the movable andstationary metal molds, a heating cylinder engaging the stationary metalmold, means for supplying a thermoplastic resin into the heatingcylinder, a screw contained in the heating cylinder for injecting moltenresin into the mold cavity, means for rotating the screw and means forreciprocating the screw in the heating cylinder, characterized in thatthe apparatus comprises a screw position detector for detecting theaxial position of the screw, a plurality of screw position settersrespectively setting predetermined screw positions along which the screwis moved, a plurality of comparators respectively comparing the outputsof the screw position setters with the output signal of the screwposition detector, a plurality of mold clamping force setters connectedto receive output signals of the respective comparators, a transferswitch for selecting one of the outputs of the mold clamping forcesetters, and an electromagnetic transfer valve energized by the outputof the transfer switch through a control device for actuating the oilpressure actuator.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing a single FIGURE shows an injection moldingmachine and electric and hydraulic circuits for operating the injectionmolding machine according to this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the accompanying drawing, the compression typeinjection molding machine 11 embodying the invention comprises a screw12 reciprocated by an injection cylinder 4 and rotated by an electric oran oil pressure motor 5. The screw 12 is contained in a heating cylinder15 having a hopper 50 at one end for charging a thermoplastic resin. Theinjection molding machine further comprises a movable metal mold 16mounted on a movable plate 17, the movable metal mold being moved towardand away from a stationary metal mold 20 supported by a stationary plate19 when a mold clamping cylinder 18 is operated so as to engage with thestationary metal mold 20 to define a mold cavity 21. The cavity 21 isdefined such that when movable metal mold 16 is inserted into thestationary metal mold 20 a compression clearance or gap δ would be lefttherebetween so that the clearance δ is closed or reduced to zero whilethe molten resin is filled into the mold cavity. A position detector 22is provided for detecting an axial position of screw 12, which may be apotentiometer, for example. There are also provided screw positionsetters 23-26 which are set with any desired screw positionsrespectively and comparators 27-30 which respectively compare the outputof the screw position detector 22 and the outputs of screw positionsetters 23-26 for producing signals for controlling the screw speed atany one of the set screw positions and the mold clamping force.

There are provided a clearance detector 31 mounted on the movable andstationary metal molds, a compression degree setter 32 which sets adesired clearance δ and a comparator 33 comparing the output ofclearance detector 31 and the set value of the clearance δ. Thecomparator 33 outputs a signal when the detected value of δ exceeds theset value of δ. A timer 34 is provided for setting the rotation time ofthe screw, that is the filling time of the molten resin. A comparator 35compares the output signal of timer 34 and a signal S representing aninterval after start of the rotation of screw 12. There are provided aplurality of mold clamping force setters 36-41 which are respectivelyinputted with the outputs of comparators 27-30, 33 and 35; a transferswitch 44 having a plurality of stationary contacts a-f which arerespectively connected to the output terminals of mold clamping forcesetters 36-41 and a movable contact which is operated manually to selecteither one of the stationary contacts a-f or automatically when eitherone of the mold clamping force setters 36-41 is set.

42 shows an electromagnetic relief valve which relieves a portion of thepressurized oil outputted from an oil pump 52 to supply oil of aconstant pressure to the mold clamping cylinder 18 via anelectromagnetic transfer valve 43 of the well known construction andcontrolled by a signal from a controller 45 supplied with the outputsignals of comparator 33 and transfer switch 44. Thus, the controller 45controls the electromagnetic transfer valve 43 in accordance with thedeviation of clearance δ and the output of one of the mold compressionforce setters 36-41, thereby applying an optimum mold clamping force tothe movable metal mold 16 during injection of the molten resin.

A plurality of injection speed setters 121a-124a and a plurality ofscrew rotation number setters 121b-124b are respectively connected inparallel to mold clamping force setters 36-40. In other words, theinjection speed setters 121a-124a and screw rotation number setters121b-124b are supplied with the output signals of comparators 27-30,respectively.

The output signals of injection speed setters 121a-123a are applied toinput terminals of an OR gate circuit 125a, while the output signals ofscrew rotation number setters 121b-124b are supplied to input terminalsof another OR gate circuit 125b. The output signals of the OR gatecircuits 125a and 125b are respectively supplied to the operatingsolenoids of electromagnetic throttle valves or flow quantity controlvalves 126 and 127. A portion of the pressurized oil outputted by an oilpump 129 is relieved by a relief valve 128 so that pressurized oil of aconstant pressure is supplied to the oil pressure motor 5 through flowquantity control valve 127 and to injection cylinder 4 through flowquantity control valve 126 and electromagnetic transfer valve 130.

Although in this embodiment, the position of the screw is fixed by oilpressure means such position fixing can also be made with mechanicalmeans.

In operation, when the filling step is completed the screw reaches theforward limit position. Then the metal molds are opened by moving themovable metal mold 16 to the left to take out the molded product. Afterthat, metal molds are partially closed while leaving a predeterminedcompression clearance δ therebetween as shown in the drawing. Forpreparing the next filling step, the molten resin is stored in a spacein the heating cylinder 15 in front of screw 12 by rotating the same.Then the screw is retracted by the cylinder 4 while being rotated by oilpressure motor 14. When the screw position detector 22 detects apredetermined screw position, not shown, the signal T is issued so thatthe electromagnetic transfer valve 130 is moved to position A to blockthe pressurized oil discharged from the injection cylinder 4.Consequently, the screw 12 continues to rotate at that predeterminedposition, whereby the molten resin is accumulated in the space in frontof the screw for an interval set by the timer 34. After elapse of a timeset by the timer 34, the electromagnetic transfer valve 130 is broughtto position B to advance the screw 12 by injection cylinder 4 so as toinject the molten resin accumulated in the space in front of the screwinto the mold cavity 21 under a high pressure. As the screw reaches apredetermined position, for example a position set by screw positionsetter 26, and during the injection stroke the mold clamping cylinder 18is actuated to clamp together the stationary and movable metal molds 20and 16 so as to decrease the clearance δ.

With the method and apparatus of this invention, at the time ofcompression molding, the stationary and movable metal molds are closedwith a predetermined compression clearance δ left therebetween, and themolten resin is filled in the mold cavity by the rotation of the screw12 or by a combination of the rotation and the advancement of the screw.Consequently, the problem of insufficient quantity of the injected resinat each shot and the problem of creating internal stress can be solved.Moreover, it is possible to increase the injected quantity by using aninjection molding machine of a relatively small capacity.

What is claimed is:
 1. A method of operating an injection compressionmolding machine comprising a stationary metal mold, a movable metalmold, an oil pressure actuator for advancing said movable metal moldtoward said stationary metal mold to form a mold cavity therebetweenwith a predetermined compression clearance δ left between said movableand stationary metal molds, a heating cylinder adapted to engage withsaid stationary metal mold, means for supplying a thermoplastic resininto said heating cylinder, a screw contained in said heating cylinderfor injecting molten resin into said mold cavity, means for rotatingsaid screw, and means for reciprocating said screw through said heatingcylinder, said method comprising the steps of:advancing said movablemetal mold toward said stationary metal mold to form the mold cavitytherebetween with the predetermined compression clearance δ left betweensaid movable and stationary metal molds; filling said molten resin insaid mold cavity by advancing said screw in an injection stroke;detecting positions of said advancing injection screw; and startingadvancement of said movable metal mold toward said stationary metal moldfor compression molding when said screw reaches a predetermined setposition during said injection stroke and before the screw has completedits stroke so as to partially reduce said compression clearance δ and toincrease pressure acting upon said molten resin injected into said moldcavity.
 2. The method according to claim 1 further comprising the stepof advancing said movable mold when said compression clearance δdeviates from a predetermined value but before said screw reaches saidpredetermined set position.
 3. The method according to claim 1 whereinsaid advancement of said screw is started by a timing completion signalissued by a timer which starts to operate when said screw reaches apredetermined retracted position.
 4. A method of operating an injectioncompression molding machine comprising a stationary metal mold, amovable metal mold, an oil pressure actuator for advancing said movablemetal mold toward said stationary metal mold to form a mold cavitytherebetweeen with a predetermined compression clearance δ left betweensaid movable and stationary metal molds, a heating cylinder adapted toengage with said stationary metal mold, means for supplying athermoplastic resin into said heating cylinder, a screw contained insaid heating cylinder for injecting molten resin into said mold cavity,means for rotating said screw, and means for reciprocating said screwthrough said heating cylinder, said method comprising the stepsof:advancing said movable metal mold toward said stationary metal moldto form said mold cavity therebetween with the predetermined compressionclearance δ left between said movable and stationary metal moldaccumulating molten resin in a space in said heating cylinder in frontof said screw by rotating said screw; injecting said accumulated moltenresin into said mold cavity by rotating said screw by advancing saidscrew in an injection stroke; detecting positions of said advancinginjection screw; and advancing said movable metal mold toward saidstationary metal mold for compression molding and to partially reducesaid compression clearance δ and to increase pressure applied to saidmolten resin injected into said mold cavity when said screw reaches apredetermined set position during said injection stroke and before thescrew has completed its stroke.
 5. The method according to claim 4wherein advancement of said screw is started at a predetermined timeafter said screw reaches a predetermined retracted position.
 6. Themethod according to claim 4 further comprising the step of advancingsaid movable mold when said compression clearance is caused to increaseby said molten resin injected into said mold cavity but before saidscrew reaches said predetermined set position.